The present invention generally relates to spectrum analysis and, more particularly, to an apparatus and method for efficiently and accurately analyzing narrow bandwidths within a wide spectral range.
The analysis of signals in terms of their frequency spectrum is an important tool which is widely used to provide information related to electrical and mechanical systems. Typically, signals are analyzed in the dc-32 MHz range. This is generally the HF frequency band containing the AM band, citizen bands, etc. In engineering applications, an analyzer may be used to test the components of a system. For example, the purity of signal sources may be observed. If a radio station is restricted to broadcasting at 1 MHz, for example, and the station must maintain a certain width in their modulation levels, an analyzer may be used to monitor the station's signal. In mechanical applications, an analyzer permits monitoring of vibration components of rotating machines associated with, for example, unbalance, worn bearings, etc.
FIG. 1 illustrates an analog analyzer made in accordance with the prior art. An antenna 10 is coupled to a mixer 15 having a local oscillator 20. Oscillator 20 is coupled to a bandpass filter 25 which is in turn coupled to a detector 27 having an audio output. Similar components may be found in a radio. Detector 27 may alternatively be coupled to a CRT to display the signal or to a computer. Bandpass filter 25 permits analysis of one signal at a time. In order to produce an amplitude vs. frequency display, a spectrum analyzer includes a sweep generator 35 coupled to local oscillator 20. Sweep generator 35 rapidly tunes the local oscillator 20 through the signal band of interest. For example, in the AM radio band, the local oscillator would be tuned through frequencies from 560 KHz to 1.6 MHz. At the same time, sweep generator 35 could drive the X-axis on a CRT while detector 27 drives the Y-axis thereby producing a visual display. If, however, the bandpass filter is made narrow to resolve the frequencies which are close together, the sweep time becomes very long. If a particular signal transmission is of short duration, the wrong part of the spectrum may be being analyzed when the transmission occurs and the signal may be missed. Such a situation may occur when the analyzer is being used in surveillance operations to detect short-lived radio transmissions.
An incoming signal may also be fed directly to an A/D converter 40 as shown in FIG. 2. A/D converter 40 may be run at a 100 MHz sampling rate. A/D converter 40 is coupled to a fast FFT 45 and a display. However, present 100 MHz A/D converters are only 8-bit converters. To match the dynamic range of the system of FIG. 1, at least 10 bits are necessary. In addition, a digital filter 50 is needed if narrow resolution is required. The amount of processing thus required would have to be done "off-line" and a relatively long time period would be required to process real time displays. In short to match the frequency coverage of the prior art, a high sampling rate is required. To match the resolution, a great deal of computing is required.
FIG. 3 illustrates another prior art embodiment. FIG. 3 illustrates a down converter which received a dc-32 MHz signal and generated a 300 KHz band of data. This 300 KHz band of data was fed to a signal processor 57 to perform FFT operation. However, the circuitry required was unwiedly and only a 300 KHz band could be analyzed.